The human genome initiative includes as one of its goals the characterization of the mouse as a model system. We have been involved in the development of a high density genetic map of the mouse by analysis of several multilocus crosses derived from interspecies or intersubspecies matings. DNAs from these mice have been typed for over 1000 loci, about half of which have also been genetically mapped in other systems. This permits us to map newly defined genes to specific positions in the genetic map and to integrate our data into composite maps of each chromosome. These studies have, most recently, resulted in the genetic mapping of genes encoding sodium channels, protein tyrosine kinases, phospholipase C subtypes, and genes involved in signal transduction. Specific map locations can be useful information since proximity to a known developmental mutation can identify such a gene as a potential candidate for the abnormal phenotype. Other studies have focussed on the organization of multigene families in the mammalian genome and on the comparative linkage relationships of homologous genes in man and mouse.